Contour correction technology is a technology that corrects the contours of an image and increases the image's sense of contrast. Contour correction technology is largely split into two; for contour correction technology there is shoot contour correction technology which attaches an overshoot and an undershoot before and after a contour element, and a shootless contour correction technology which steepens the change in the brightness signal in a portion of the contour.
Additionally, in shootless contour correction technology, there is a method in which one of the values for a pixel (for example, the brightness) near the pixel to be corrected is selected, and the selected value is made into the value of the aforementioned pixel to be corrected (for example, see Patent Document 1 and 2).
FIG. 1 is a functional block diagram of the image processing apparatus in the Patent Document 1 above.
An image processing apparatus 800 includes a holding unit 810, a differentiating unit 820, a coring unit 830, a comparator unit 840 and a signal selection unit 850.
The holding unit 810 acquires and holds a picture signal G1.
The differentiating unit 820 performs differentiation on the image signal G1 and outputs a primary differential signal G2 which indicates the differentiated result.
The coring unit 830 suppresses small oscillations of the primary differential signal G2 and smoothes the waveform.
The comparator 840 ranks the levels of ascent and decline in the primary differential signal G2, the waveform of which is smoothed by the coring unit 830.
The signal selection unit 850 selects, from a plurality of values included in the image signal G1 which is held in the holding unit 810, a value according to the rank outputted from the comparator 840. As a result, the signal selection unit 850 generates and outputs a corrected image signal G9.
The image processing apparatus 800 obtains the image signal G1, and utilizes the primary differential signal G2, which is generated by differentiating the image signal G1, as a contour correction control signal. Then the image processing apparatus 800 corrects the image signal G1 according to the primary differential signal G2, i.e. the contour correction control signal, and generates the corrected image signal G9.
FIG. 2 is a figure for presenting the signal generated by the image processing apparatus 800 in the Patent Document 1 above.
For example, the image signal G1 is a signal that indicates a brightness Y for every horizontal pixel position X. Here, at a horizontal pixel position X, the smaller the brightness Y, the blacker the pixel at the position, and the larger a brightness Y, the whiter the pixel at the position. Accordingly, the image signal G1 shown in FIG. 2 expresses the contours of a black image and a white image.
As shown in FIG. 2, the value of each horizontal pixel coordinate X indicated by the primary differential signal G2 increases accordingly as X heads from the horizontal pixel coordinate X on the left side towards the horizontal pixel coordinate X on the right side, and after reaching its greatest value at the center of the contour Xt, declines accordingly.
The image processing apparatus 800 selects, for every horizontal pixel coordinate X, the picture signal G1's brightness Y at a horizontal pixel coordinate X distanced by a distance according to the value that the primary differential signal G2 above indicates, and then generates a corrected image signal G9 as shown in FIG. 2.
In the corrected image signal G9 generated in this way, the gradient of brightness Y is larger than the gradient of the image signal G1, and the contour is sharpened.
FIG. 3 is a functional block diagram of an image processing apparatus in the second Patent Document above.
The image processing apparatus 900 includes a delay unit 910, a primary differentiating unit 920, an absolute value computing unit 930, a secondary differentiating unit 940 and a time axis modulating unit 950.
The delay unit 910 obtains the image signal T1, and in order to align the average delay time and the timing of the time axis modulation unit 950, delays the image signal T1.
The primary differentiating unit 920 performs differentiation on the image signal T1 delayed by the delay unit 910, and outputs the result as a primary differential signal.
The absolute value computing unit 930 takes the absolute value of the values indicated by the primary differential signal and outputs the results as an absolute value signal.
The secondary differentiating unit 940 performs differentiation on the absolute value signal and outputs the result as a secondary differential signal T2.
The time axis modulation unit 950 includes a memory and stores the image signal T1 in the memory. Then, the time axis modulation unit 950 selects a value from a plurality of values (for example the brightness) displayed by the image signal T1, according to the secondary differential signal T2 outputted from the secondary differentiating unit 940. As a result, the time axis modulation unit 950 generates and outputs the corrected image signal T9.
The image processing apparatus 900 obtains the image signal T1 and utilizes the secondary differential signal T2 as a contour correction control signal, the secondary differential signal T2 being generated by secondarily differentiating the image signal T1. Then, the image processing apparatus 900 corrects the image signal T1 according to the secondary differential signal T2, which is the contour correction control signal, and generates a corrected image signal T9.
FIG. 4 is a figure for describing the signal generated by the image processing apparatus 900 in the Patent Document 2 above.
For instance, the image signal T1 is a signal that indicates the brightness Y for every horizontal pixel coordinate X. Here, at a horizontal pixel position X, the smaller the brightness Y, the blacker the pixel at the position, and the larger the brightness Y, the whiter the pixel at that position. Therefore, the image signal T1 shown in FIG. 4 shows the contours of a black-colored image and a white-colored image.
The value indicated by the secondary differential signal T2 repeatedly increases and declines as it heads from the left side of the horizontal pixel coordinate X to the right side of the horizontal pixel coordinate X.
The image processing apparatus 900 selects, for every horizontal pixel coordinate X, a brightness Y of the picture signal T1 at the horizontal pixel coordinate X, distanced only by a distance according to the value that the secondary differential signal T2 indicates, and generates a corrected image signal T9 as shown in FIG. 4.
In the corrected image signal T9 generated in this way, the gradient of the brightness Y is larger than that of the image signal T1 and the contours are sharpened. Further, the corrected image signal T9 shows sharper contours than the corrected image signal G9, which is generated with the image processing apparatus 800 in the above Patent Document 1.    Patent Reference 1: Japanese Patent Laid-Open No. 2000-32298 bulletin (Page 5, FIG. 1)    Patent Reference 2: Japanese Patent Laid-Open No. 4-6960 bulletin (Page 7, FIG. 1)